1. Field of the Invention
This invention relates generally to methods, systems and apparatus for managing digital communication systems. More specifically, this invention relates to adaptive control of various transmission parameters, including but not limited to maximum transmit power spectral density, maximum aggregate transmission power, transmission band preference, minimum and maximum receiver margin, frequency-dependent bit-loading and power controls and/or bit-loading restrictions in communication systems such as Digital Subscriber Line (DSL) systems.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice-band analog communication. In particular, asymmetric DSL (ADSL) can adjust to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions as determined during training and initialization of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line. The adaptive assignment can be continued during live data transmission on channels or lines that vary with time using a process often referred to as “bit-swapping” that uses a secure relatively low-speed reverse channel to inform the transmitter of assignment changes.
Impulse noise, other noise and other sources of error can substantially impact the accuracy of data transmitted by DSL and other communications systems. Various techniques have been developed for reducing, avoiding and/or repairing the damage done to data by such error during transmission. These error reduction/avoidance/repair techniques have performance costs for a communication system in which they are used. As is well known in the art, inadequate power transmission levels lead to errors because the transmission power is not high enough to overcome noise and other interference in a given channel. These errors lead to lost data and/or the need for re-transmission of data, sometimes multiple times. To prevent such errors, systems utilize extra transmission power that results in margins above a known or calculated signal-to-noise ratio (SNR) that assures compliance with an acceptable error rate.
Excessively high power transmission levels, however, lead to other problems. For example, use of transmission power above necessary levels means that the communication system is operated more expensively, to the detriment of all users. In addition, one or more lines' use of excessive transmission power can generate strong crosstalk problems and interference in nearby lines. Crosstalk is unwanted interference and/or signal noise electromagnetically passed between lines that share the same or adjacent binders. Crosstalk can be categorized as far-end crosstalk (FEXT) or near-end crosstalk (NEXT). FEXT is particularly detrimental in certain loop configurations with different lengths. One such situation is when a first DSL service (for example, a DSL loop or line) is deployed from a central office (CO) and a second DSL service is deployed from a remote terminal (RT), a service access interface (SAI), an optical network unit (ONU), a pedestal or any other location outside a CO. In such situations, FEXT from the CO-deployed service may cause considerable degradation to a service deployed from the non-CO location. Another strong FEXT situation arises with short to medium loop lengths, when a short line can cause strong interference into the receiver of a longer line. One such situation arises when VDSL service is deployed on loops with different lengths, in which case the FEXT crosstalk interference can be particularly strong in the upstream direction. NEXT can have a damaging effect in DSL configurations where there is some overlap between the bands used for transmission in the downstream and upstream direction, or where there is signal leakage from a downstream transmitter to an upstream receiver or vice versa.
Systems, devices, methods and techniques that allow users to adjust and adapt transmission power margin(s), power spectral densities, and the like dynamically to changing DSL environmental and operational situations would represent a significant advancement in the field of DSL operation. Moreover, monitoring and evaluation of the power, margins, etc. used in the DSL environment and operation by an independent entity can assist, guide and (in some cases) control users' activities and equipment, and likewise would represent a significant advancement in the field of DSL operation.